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SubjectRe: SCSI access creating lost time
On Sun, 7 Mar 1999, Doug Ledford wrote:

>The races include do_scsi_cmnd() vs. scsi_done() vs. scsi_times_out().

The harm is not the locking overkill, but instead the fact that during the
locked-atomic region we have the cpu __cli()ed.

And the _only_ reason we need the cpu clied in order to do the locking
safe, is that all places are currently looping infinitely on their lock
(in this case a -spin-lock) and not always-taking a fail-safe path.

I think instead that all places that could run in a irq handler should do:

struct semaphore scsi_sem = MUTEX;

scsi_done()
{
if (down_trylock(&scsi_sem))
{
queue_task();
return;
}
/* lock held */
up(&scsi_sem);
}
and places that can be recalled only from normal kernel context (I
think none in the scsi code though ;) should do:

down()
/* Critical section */
up();
That way we would _never_ need the cpu clied.

Comments?

Andrea Arcangeli


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