On Thu, Jun 29, 2017 at 03:53:22PM -0400, Tejun Heo wrote:
> Hello, Paul.
>
> On Thu, Jun 29, 2017 at 11:10:57AM -0700, Paul E. McKenney wrote:
> > If this code fragment doesn't deadlock, then CPU 0's spin_unlock_wait()
> > must have executed before CPU 1's spin_lock(). However, even on x86,
> > CPU 0's prior writes can be reordered with its subsequent reads, which
> > means that r1 == 0 is possible, which means that the above condition
> > could hold, even on x86.
>
> I see. Ah, that's a mind bender.
It has indeed been providing at least its share of entertainment over
the past little while. ;-)
> > One of the uses of spin_unlock_wait() is in ata_scsi_cmd_error_handler()
> > in the file drivers/ata/libata-eh.c. Your commit ad9e27624479b
> > ("libata-eh-fw: update ata_scsi_error() for new EH") last touched it,
> > though it predates that commit.
> >
> > My question to you is whether the code in ata_scsi_cmd_error_handler()
> > needs release semantics. If it does, my recommendation is to replace
> > the spin_unlock_wait(ap->lock) with this (adding the needed curly braces,
> > of course):
> >
> > spin_lock(ap->lock);
> > spin_unlock(ap->lock);
> >
> > If the code only needs acquire semantics, no change required.
> >
> > If your code requires release semantics, and there is some reason why
> > my suggested replacement above is a bad idea, please let me know!
>
> That part of the code should be dead now. I don't think we no longer
> have any driver which doesn't have error handler set. I should rip
> out that if/else. Also, ACQUIRE semantics should be enough there.
> Nothing changes from the EH side there.
It looks like we actually might get rid of spin_unlock_wait entirely.
But how about if I just pull the spin_lock_irqsave() before the "if"
and the spin_lock_irqrestore() after the "if"? Same effect, only
difference is that the "if" and the "ap->eh_tries = ATA_EH_MAX_TRIES"
end up under the lock, and I bet that you won't be able to measure
the difference. (Please see below.)
I will do this because I just now happened to be editing that file on
my "eradicate spin_unlock_wait()" quest, but can easily rework the
patch as desired. If you want something different, just let me know!
Thanx, Paul
------------------------------------------------------------------------
commit 39a15ef3b324b08606953d519e9bc538318f3c15
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Date: Thu Jun 29 13:10:47 2017 -0700
drivers/ata: Replace spin_unlock_wait() with lock/unlock pair
There is no agreed-upon definition of spin_unlock_wait()'s semantics,
and it appears that all callers could do just as well with a lock/unlock
pair. This commit therefore eliminates the spin_unlock_wait() call and
associated else-clause and hoists the then-clause's lock and unlock out of
the "if" statement. This should be safe from a performance perspective
because according to Tejun there should be few if any drivers that don't
set their own error handler.
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: <linux-ide@xxxxxxxxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Alan Stern <stern@xxxxxxxxxxxxxxxxxxx>
Cc: Andrea Parri <parri.andrea@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
diff --git a/drivers/ata/libata-eh.c b/drivers/ata/libata-eh.c
index ef68232b5222..779f6f18c1f4 100644
--- a/drivers/ata/libata-eh.c
+++ b/drivers/ata/libata-eh.c
@@ -645,12 +645,11 @@ void ata_scsi_cmd_error_handler(struct Scsi_Host *host, struct ata_port *ap,
* completions are honored. A scmd is determined to have
* timed out iff its associated qc is active and not failed.
*/
+ spin_lock_irqsave(ap->lock, flags);
if (ap->ops->error_handler) {
struct scsi_cmnd *scmd, *tmp;
int nr_timedout = 0;
- spin_lock_irqsave(ap->lock, flags);
-
/* This must occur under the ap->lock as we don't want
a polled recovery to race the real interrupt handler
@@ -700,12 +699,11 @@ void ata_scsi_cmd_error_handler(struct Scsi_Host *host, struct ata_port *ap,
if (nr_timedout)
__ata_port_freeze(ap);
- spin_unlock_irqrestore(ap->lock, flags);
/* initialize eh_tries */
ap->eh_tries = ATA_EH_MAX_TRIES;
- } else
- spin_unlock_wait(ap->lock);
+ }
+ spin_unlock_irqrestore(ap->lock, flags);
}
EXPORT_SYMBOL(ata_scsi_cmd_error_handler);
--
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