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

[John Baldwin]

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.

-- 
John Baldwin