Re: [PATCH v2] scsi: core: pair EH runtime PM get and put

[Alan Stern]

On Mon, Jul 27, 2026 at 01:56:15PM +0800, Hongjie Fang wrote:
> shost->eh_noresume is currently consulted twice in one error handling
> iteration: once before scsi_autopm_get_host() and once again before
> scsi_autopm_put_host().
> 
> That is racy when a PM-triggered error path flips shost->eh_noresume while
> the SCSI EH thread is still running.
> 
> The problem flow looks like this:
> PM path
>   ufshcd_set_dev_pwr_mode()
>     shost->eh_noresume = 1
>     ufshcd_execute_start_stop  <-- trigger EH
>     ...
>     shost->eh_noresume = 0
> 
> EH path
>   scsi_error_handler()
>     if (!shost->eh_noresume)
>       scsi_autopm_get_host()  <-- skipped
>     ...
>     if (!shost->eh_noresume)
>        scsi_autopm_put_host()  <-- executed later
> 
> In that case one EH iteration can skip autoresume on entry and still drop
> a runtime PM reference on exit. That leaves an unmatched runtime PM put
> and can trigger a runtime PM usage count underflow.
> 
> Fix this by snapshotting shost->eh_noresume once per EH iteration and
> using that snapshot for both runtime PM get and put decisions.
> 
> Fixes: ae0751ffc77e ("[SCSI] add flag to skip the runtime PM calls on the host")
> Signed-off-by: Hongjie Fang <hongjiefang@xxxxxxxxxxxx>
> ---
>  drivers/scsi/scsi_error.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/scsi/scsi_error.c b/drivers/scsi/scsi_error.c
> index 147127fb4db9..a216f56044d9 100644
> --- a/drivers/scsi/scsi_error.c
> +++ b/drivers/scsi/scsi_error.c
> @@ -2342,6 +2342,7 @@ static void scsi_unjam_host(struct Scsi_Host *shost)
>  int scsi_error_handler(void *data)
>  {
>  	struct Scsi_Host *shost = data;
> +	bool eh_noresume;
>  
>  	/*
>  	 * We use TASK_INTERRUPTIBLE so that the thread is not
> @@ -2383,7 +2384,8 @@ int scsi_error_handler(void *data)
>  		 * what we need to do to get it up and online again (if we can).
>  		 * If we fail, we end up taking the thing offline.
>  		 */
> -		if (!shost->eh_noresume && scsi_autopm_get_host(shost) != 0) {
> +		eh_noresume = shost->eh_noresume;

You should be aware that simply reading a variable does not snapshot its 
value unless the read is protected by a lock or something else.  In this 
case, the compiler is allowed to eliminate the eh_noresume variable and 
use shost->eh_noresume instead, both here and below, under the 
assumption that shost->eh_noresume doesn't change in the meantime.

To truly snapshot the the value in a way that's immune to changes from 
other threads, you have to use READ_ONCE():

		eh_noresume = READ_ONCE(shost->eh_noresume);

The compiler is then not allowed to assume that shost->eh_noresume 
remains unchanged later on.  This is all explained (along with a _lot_ 
of other material -- search for "READ_ONCE") in 
Documentation/memory-barriers.txt.

Alan Stern

> +		if (!eh_noresume && scsi_autopm_get_host(shost) != 0) {
>  			SCSI_LOG_ERROR_RECOVERY(1,
>  				shost_printk(KERN_ERR, shost,
>  					     "scsi_eh_%d: unable to autoresume\n",
> @@ -2407,7 +2409,7 @@ int scsi_error_handler(void *data)
>  		 * which are still online.
>  		 */
>  		scsi_restart_operations(shost);
> -		if (!shost->eh_noresume)
> +		if (!eh_noresume)
>  			scsi_autopm_put_host(shost);
>  	}
>  	__set_current_state(TASK_RUNNING);
> -- 
> 2.25.1
>