git: 35547df5c786 - main - Call wakeup() with the lock held to avoid missed wakeup races.

[John Baldwin]

On 8/11/21 2:38 PM, Luiz Otavio O Souza wrote:
> On Wed, Aug 11, 2021 at 3:49 PM John Baldwin <jhb at freebsd.org> wrote:
>>
>> On 8/10/21 3:41 PM, Scott Long wrote:
>>> The branch main has been updated by scottl:
>>>
>>> URL: https://cgit.FreeBSD.org/src/commit/?id=35547df5c78653b2da030f920323c0357056099f
>>>
>>> commit 35547df5c78653b2da030f920323c0357056099f
>>> Author:     Scott Long <scottl at FreeBSD.org>
>>> AuthorDate: 2021-08-10 22:36:38 +0000
>>> Commit:     Scott Long <scottl at FreeBSD.org>
>>> CommitDate: 2021-08-10 22:36:38 +0000
>>>
>>>       Call wakeup() with the lock held to avoid missed wakeup races.
>>>
>>>       Submitted by: luiz
>>>       Sponsored by: Rubicon Communications, LLC ("Netgate")
>>> ---
>>>    sys/dev/sdhci/sdhci.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/sys/dev/sdhci/sdhci.c b/sys/dev/sdhci/sdhci.c
>>> index d075c2e05000..573e6949b57e 100644
>>> --- a/sys/dev/sdhci/sdhci.c
>>> +++ b/sys/dev/sdhci/sdhci.c
>>> @@ -2078,8 +2078,8 @@ sdhci_generic_release_host(device_t brdev __unused, device_t reqdev)
>>>        /* Deactivate led. */
>>>        WR1(slot, SDHCI_HOST_CONTROL, slot->hostctrl &= ~SDHCI_CTRL_LED);
>>>        slot->bus_busy--;
>>> -     SDHCI_UNLOCK(slot);
>>>        wakeup(slot);
>>> +     SDHCI_UNLOCK(slot);
>>>        return (0);
>>>    }
>>
>> Hmm, how does this avoid a race?  The sleep is checking bus_busy under
>> the lock and should never see a stale value and go back to sleep after
>> the wakeup has occurred:
>>
>>          SDHCI_LOCK(slot);
>>          while (slot->bus_busy)
>>                  msleep(slot, &slot->mtx, 0, "sdhciah", 0);
>>          slot->bus_busy++;
>>          /* Activate led. */
>>          WR1(slot, SDHCI_HOST_CONTROL, slot->hostctrl |= SDHCI_CTRL_LED);
>>          SDHCI_UNLOCK(slot);
>>
>> Dropping the lock before wakeup() is a tiny optimization that avoids
>> having the second thread wakeup and immediately block on the lock before
>> it has been released by the first thread.
>>
> 
> 'race' is probably wrong here.  this change will prevent a second
> thread from taking the bus before you call wakeup() - poking all other
> threads unnecessarily.

This change does not prevent that.  The other thread and the thread that
are awakened will race with each other to acquire the lock.  wakeup()
doesn't do any sort of explicit lock handoff to the thread being awakened
and it's just as likely for a thread not yet asleep to acquire the lock as
for the thread being awakened to acquire the lock.  If you have observed
thundering herd problems with this wakeup() then you might want to change
it to wakeup_one().

-- 
John Baldwin