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Re: [PATCHv3] PowerPC: Fix a race condition when eliding a lock
- From: "Paul E. Murphy" <murphyp at linux dot vnet dot ibm dot com>
- To: libc-alpha at sourceware dot org, Adhemerval Zanella <adhemerval dot zanella at linaro dot org>
- Date: Tue, 01 Sep 2015 16:24:41 -0500
- Subject: Re: [PATCHv3] PowerPC: Fix a race condition when eliding a lock
- Authentication-results: sourceware.org; auth=none
- References: <55D742D3 dot 9050600 at redhat dot com> <1440439895-11812-1-git-send-email-tuliom at linux dot vnet dot ibm dot com> <1441136302 dot 5089 dot 182 dot camel at otta> <55E60E88 dot 50104 at linaro dot org>
On 09/01/2015 03:46 PM, Adhemerval Zanella wrote:
> Indeed the 'odd' comment does not make sense and we should just remove it
> (I misread texasr definition). My initial idea was define some codes
> that set the persistent failures and some that do not. I think I best
> approach would be:
>
> /* tabort will set TEXASR(0:31) = ((_ABORT_LOCK_BUSY & 0xff) << 24) | 0x1
> and the TEXASR persistent bit is bit 25 (32-7). Only the syscall
> code means a persistent error that should trigger a default lock
> acquisition. */
> #define _ABORT_SYSCALL 0x1 /* Syscall issued. */
> #define _ABORT_LOCK_BUSY 0x2 /* Lock already used. */
> #define _ABORT_NESTED_TRYLOCK 0x4 /* Write operation in trylock. */
The kernel defines several abort codes, we'll want to work with them, or
recycle them as needed.
I'm not convinced any of the existing codes should be non-persistent:
A pthread_mutex_trylock attempt within an elided pthread_mutex_lock is
guaranteed to fail try_tbegin times if there is no contention on the lock.
Aborts get increasingly expensive as you increase the amount of speculative
execution.
A busy lock likely indicates contention in the critical section which
does not benefit from elision, I'd err on the side of a persistent
failure.