Re: [PATCH 2/3] smp/ipi:Clarify ambiguous comments around deadlockscenarios in smp_call_function variants.

[Thomas Gleixner]

On Sat, 6 Jul 2013, Preeti U Murthy wrote:

> Hi Wang,
> 
> On 07/06/2013 11:42 AM, Wang YanQing wrote:
> > On Fri, Jul 05, 2013 at 09:57:11PM +0530, Preeti U Murthy wrote:
> >> Elaborate on when deadlocks can occur when a call is made to
> >> smp_call_function_single() and its friends. This avoids ambiguity about
> >> when to use these calls.
> >>
> >> +	 * 2. wait = 0: This function could be called from an interrupt
> >> +	 * context, and can get blocked on the csd_lock(csd) below in
> >> +	 * "non wait cases".
> >> +	 * This is because the percpu copy of csd of this_cpu is used
> >> +	 * in non wait cases. Under such circumstances, if the previous caller
> >> +	 * of this function who got preempted by this interrupt has already taken
> >> +	 * the lock under non wait condition, it will result in deadlock.
> >> +	 *
> > 
> > No, it will not cause deadlock, it is not mutex lock,  it is busy wait, so
> > when the CSD_FLAG_LOCK be cleared, the code will go on running.
> 
> A deadlock might not result, but a potential long wait in an interrupt
> context could result if the source cpu got preempted by an interrupt
> between  csd_lock(csd) and generic_exec_single(), where it actually
> sends an ipi to the target cpu.

See https://lkml.org/lkml/2013/7/5/183 and the related thread for real
deadlock scenarios.
 
Thanks,

	tglx
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/