Re: [Xen-devel] [PATCH 1/2] rwlock: add per-cpu reader-writer locks

["Marcos E. Matsunaga" <Marcos.Matsunaga@xxxxxxxxxx>]

Hi Malcolm,

I tried your patches against staging yesterday and as soon as I started 
a guest, it panic. I have lock_profile enabled and applied your patches 
against:
6f04de658574833688c3f9eab310e7834d56a9c0 x86: cleanup of early cpuid 
handling


(XEN) HVM1 save: CPU
(XEN) HVM1 save: PIC
(XEN) HVM1 save: IOAPIC
(XEN) HVM1 save: LAPIC
(XEN) HVM1 save: LAPIC_REGS
(XEN) HVM1 save: PCI_IRQ
(XEN) HVM1 save: ISA_IRQ
(XEN) HVM1 save: PCI_LINK
(XEN) HVM1 save: PIT
(XEN) HVM1 save: RTC
(XEN) HVM1 save: HPET
(XEN) HVM1 save: PMTIMER
(XEN) HVM1 save: MTRR
(XEN) HVM1 save: VIRIDIAN_DOMAIN
(XEN) HVM1 save: CPU_XSAVE
(XEN) HVM1 save: VIRIDIAN_VCPU
(XEN) HVM1 save: VMCE_VCPU
(XEN) HVM1 save: TSC_ADJUST
(XEN) HVM1 restore: CPU 0
[  394.163143] loop: module loaded
(XEN) Assertion 'rw_is_locked(&t->lock)' failed at grant_table.c:215
(XEN) ----[ Xen-4.7-unstable  x86_64  debug=y  Tainted:    C ]----
(XEN) CPU:    0
(XEN) RIP:    e008:[<ffff82d08011019e>] do_grant_table_op+0x63f/0x2e04
(XEN) RFLAGS: 0000000000010246   CONTEXT: hypervisor (d0v0)
(XEN) rax: 0000000000000000   rbx: ffff83400f9dc9e0   rcx: 0000000000000000
(XEN) rdx: 0000000000000001   rsi: ffff82d080342b10   rdi: ffff83400819b784
(XEN) rbp: ffff8300774ffef8   rsp: ffff8300774ffdf8   r8: 0000000000000002
(XEN) r9:  0000000000000002   r10: 0000000000000002   r11: 00000000ffffffff
(XEN) r12: 0000000000000000   r13: 0000000000000000   r14: ffff83400819b780
(XEN) r15: ffff83400f9d0000   cr0: 0000000080050033   cr4: 00000000001526e0
(XEN) cr3: 000001007f613000   cr2: ffff8800746182b8
(XEN) ds: 0000   es: 0000   fs: 0000   gs: 0000   ss: e010   cs: e008
(XEN) Xen stack trace from rsp=ffff8300774ffdf8:
(XEN)    ffff8300774ffe08 ffff82d000000000 ffff8300774ffef8 ffff82d08017fc9b
(XEN)    ffff82d080342b28 ffff83400f9d8600 ffff82d080342b10 0000000000000000
(XEN)    ffff83400f9dca20 ffff832100000000 ffff834008188000 0000000000000001
(XEN)    00000001772ee000 ffff8801e98d03e0 ffff8300774ffe88 0000000000000000
(XEN)    0000000000000000 ffff8300774fff18 00000021d0269c10 000000000001001a
(XEN)    ffffffff00000001 0000000000000000 0000000000000246 00007ff7de45a407
(XEN)    0000000000000100 00007ff7de45a407 0000000000000033 ffff8300772ee000
(XEN)    ffff8801eb0e3c00 ffff880004bf57e8 ffff8801e98d03e0 ffff8801eb0a5938
(XEN)    00007cff88b000c7 ffff82d08023d952 ffffffff8100128a 0000000000000014
(XEN)    0000000000000000 0000000000000001 ffff8801f6e18388 ffffffff81d3d740
(XEN)    ffff8801efb7bd40 ffff88000542e780 0000000000000282 0000000000000000
(XEN)    ffff8801e98d03a0 ffff8801efe07000 0000000000000014 ffffffff8100128a
(XEN)    0000000000000001 ffff8801e98d03e0 0000000000000000 0001010000000000
(XEN)    ffffffff8100128a 000000000000e033 0000000000000282 ffff8801efb7bce0
(XEN)    000000000000e02b 0000000000000000 0000000000000000 0000000000000000
(XEN)    0000000000000000 0000000000000000 ffff8300772ee000 0000000000000000
(XEN)    0000000000000000
(XEN) Xen call trace:
(XEN)    [<ffff82d08011019e>] do_grant_table_op+0x63f/0x2e04
(XEN)    [<ffff82d08023d952>] lstar_enter+0xe2/0x13c
(XEN)
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Assertion 'rw_is_locked(&t->lock)' failed at grant_table.c:215
(XEN) ****************************************
(XEN)
(XEN) Manual reset required ('noreboot' specified)


Thanks for your help.

On 11/03/2015 12:58 PM, Malcolm Crossley wrote:
> Per-cpu read-write locks allow for the fast path read case to have low overhead
> by only setting/clearing a per-cpu variable for using the read lock.
> The per-cpu read fast path also avoids locked compare swap operations which can
> be particularly slow on coherent multi-socket systems, particularly if there is
> heavy usage of the read lock itself.
>
> The per-cpu reader-writer lock uses a global variable to control the read lock
> fast path. This allows a writer to disable the fast path and ensure the readers
> use the underlying read-write lock implementation.
>
> Once the writer has taken the write lock and disabled the fast path, it must
> poll the per-cpu variable for all CPU's which have entered the critical section
> for the specific read-write lock the writer is attempting to take. This design
> allows for a single per-cpu variable to be used for read/write locks belonging
> to seperate data structures as long as multiple per-cpu read locks are not
> simultaneously held by one particular cpu. This also means per-cpu
> reader-writer locks are not recursion safe.
>
> Slow path readers which are unblocked set the per-cpu variable and drop the
> read lock. This simplifies the implementation and allows for fairness in the
> underlying read-write lock to be taken advantage of.
>
> There may be slightly more overhead on the per-cpu write lock path due to
> checking each CPUs fast path read variable but this overhead is likely be hidden
> by the required delay of waiting for readers to exit the critical section.
> The loop is optimised to only iterate over the per-cpu data of active readers
> of the rwlock.
>
> Signed-off-by: Malcolm Crossley <malcolm.crossley@xxxxxxxxxx>
> ---
>   xen/common/spinlock.c        | 32 ++++++++++++++++++++++++++++++++
>   xen/include/asm-arm/percpu.h |  5 +++++
>   xen/include/asm-x86/percpu.h |  6 ++++++
>   xen/include/xen/percpu.h     |  4 ++++
>   xen/include/xen/spinlock.h   | 37 +++++++++++++++++++++++++++++++++++++
>   5 files changed, 84 insertions(+)
>
> diff --git a/xen/common/spinlock.c b/xen/common/spinlock.c
> index 7f89694..a526216 100644
> --- a/xen/common/spinlock.c
> +++ b/xen/common/spinlock.c
> @@ -492,6 +492,38 @@ int _rw_is_write_locked(rwlock_t *lock)
>       return (lock->lock == RW_WRITE_FLAG); /* writer in critical section? */
>   }
>   
> +void percpu_write_lock(rwlock_t **per_cpudata, bool_t *writer_activating,
> +               rwlock_t *rwlock)
> +{
> +    int cpu;
> +    cpumask_t active_readers;
> +
> +    cpumask_copy(&active_readers, &cpu_online_map);
> +    /* First take the write lock to protect against other writers */
> +    write_lock(rwlock);
> +
> +    /* Now set the global variable so that readers start using read_lock */
> +    *writer_activating = true;
> +    smp_mb();
> +
> +    /* Check if there are any percpu readers in progress on our rwlock*/
> +    do
> +    {
> +        for_each_cpu(cpu, &active_readers)
> +        {
> +            /* Remove any percpu readers not contending
> +             * from our check mask */
> +            if ( per_cpu_ptr(per_cpudata, cpu) != rwlock )
> +                cpumask_clear_cpu(cpu, &active_readers);
> +        }
> +        /* Check if we've cleared all percpu readers */
> +        if ( cpumask_empty(&active_readers) )
> +            break;
> +        /* Give the coherency fabric a break */
> +        cpu_relax();
> +    } while ( 1 );
> +}
> +
>   #ifdef LOCK_PROFILE
>   
>   struct lock_profile_anc {
> diff --git a/xen/include/asm-arm/percpu.h b/xen/include/asm-arm/percpu.h
> index 71e7649..c308a56 100644
> --- a/xen/include/asm-arm/percpu.h
> +++ b/xen/include/asm-arm/percpu.h
> @@ -27,6 +27,11 @@ void percpu_init_areas(void);
>   #define __get_cpu_var(var) \
>       (*RELOC_HIDE(&per_cpu__##var, READ_SYSREG(TPIDR_EL2)))
>   
> +#define per_cpu_ptr(var, cpu)  \
> +    (*RELOC_HIDE(&var, __per_cpu_offset[cpu]))
> +#define __get_cpu_ptr(var) \
> +    (*RELOC_HIDE(&var, READ_SYSREG(TPIDR_EL2)))
> +
>   #define DECLARE_PER_CPU(type, name) extern __typeof__(type) per_cpu__##name
>   
>   DECLARE_PER_CPU(unsigned int, cpu_id);
> diff --git a/xen/include/asm-x86/percpu.h b/xen/include/asm-x86/percpu.h
> index 604ff0d..51562b9 100644
> --- a/xen/include/asm-x86/percpu.h
> +++ b/xen/include/asm-x86/percpu.h
> @@ -20,4 +20,10 @@ void percpu_init_areas(void);
>   
>   #define DECLARE_PER_CPU(type, name) extern __typeof__(type) per_cpu__##name
>   
> +#define __get_cpu_ptr(var) \
> +    (*RELOC_HIDE(var, get_cpu_info()->per_cpu_offset))
> +
> +#define per_cpu_ptr(var, cpu)  \
> +    (*RELOC_HIDE(var, __per_cpu_offset[cpu]))
> +
>   #endif /* __X86_PERCPU_H__ */
> diff --git a/xen/include/xen/percpu.h b/xen/include/xen/percpu.h
> index abe0b11..c896863 100644
> --- a/xen/include/xen/percpu.h
> +++ b/xen/include/xen/percpu.h
> @@ -16,6 +16,10 @@
>   /* Preferred on Xen. Also see arch-defined per_cpu(). */
>   #define this_cpu(var)    __get_cpu_var(var)
>   
> +#define this_cpu_ptr(ptr)    __get_cpu_ptr(ptr)
> +
> +#define get_per_cpu_var(var)  (per_cpu__##var)
> +
>   /* Linux compatibility. */
>   #define get_cpu_var(var) this_cpu(var)
>   #define put_cpu_var(var)
> diff --git a/xen/include/xen/spinlock.h b/xen/include/xen/spinlock.h
> index fb0438e..f929f1b 100644
> --- a/xen/include/xen/spinlock.h
> +++ b/xen/include/xen/spinlock.h
> @@ -3,6 +3,7 @@
>   
>   #include <asm/system.h>
>   #include <asm/spinlock.h>
> +#include <xen/stdbool.h>
>   
>   #ifndef NDEBUG
>   struct lock_debug {
> @@ -261,4 +262,40 @@ int _rw_is_write_locked(rwlock_t *lock);
>   #define rw_is_locked(l)               _rw_is_locked(l)
>   #define rw_is_write_locked(l)         _rw_is_write_locked(l)
>   
> +static inline void percpu_read_lock(rwlock_t **per_cpudata, bool_t *writer_activating,
> +               rwlock_t *rwlock)
> +{
> +    /* Indicate this cpu is reading */
> +    this_cpu_ptr(per_cpudata) = rwlock;
> +    smp_mb();
> +    /* Check if a writer is waiting */
> +    if ( unlikely(*writer_activating) )
> +    {
> +        /* Let the waiting writer know we aren't holding the lock */
> +        this_cpu_ptr(per_cpudata) = NULL;
> +        /* Wait using the read lock to keep the lock fair */
> +        read_lock(rwlock);
> +        /* Set the per CPU data again and continue*/
> +        this_cpu_ptr(per_cpudata) = rwlock;
> +        /* Drop the read lock because we don't need it anymore */
> +        read_unlock(rwlock);
> +    }
> +}
> +
> +static inline void percpu_read_unlock(rwlock_t **per_cpudata)
> +{
> +    this_cpu_ptr(per_cpudata) = NULL;
> +    smp_wmb();
> +}
> +
> +/* Don't inline percpu write lock as it's a complex function */
> +void percpu_write_lock(rwlock_t **per_cpudata, bool_t *writer_activating,
> +               rwlock_t *rwlock);
> +
> +static inline void percpu_write_unlock(bool_t *writer_activating, rwlock_t 
> *rwlock)
> +{
> +    *writer_activating = false;
> +    write_unlock(rwlock);
> +}
> +
>   #endif /* __SPINLOCK_H__ */
--

Regards,

Marcos Eduardo Matsunaga

Oracle USA
Linux Engineering

“The statements and opinions expressed here are my own and do not
necessarily represent those of Oracle Corporation.”


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