Hi Sebastian, On Tue, 2020-06-09 at 17:47 +0200, Sebastian Andrzej Siewior wrote: > On 2020-06-04 15:51:14 [-0500], Tom Zanussi wrote: > > > > > > Hi, This patch introduced a regression in our kernel > > > (v4.19.124-rt53-rebase), It occurs when we're jumping to crush > > > kernel > > > using kexec, in the initialization of the emmc driver. > > > I'm still debugging the root cause, but I thought of mentioning > > > this > > > in the mailing list if you have any idea why this could occur. > > > The issue doesn't happen on normal boot, only when I specifically > > > crash the kernel into the crash kernel. > > > Thanks, > > > Ramon. > > > > I'm not very familiar with crashing the kernel into the crash > > kernel. > > Can you explain in enough detail how to set things up to reproduce > > this > > and how to trigger it? Does it happen every time? > > > > > From looking at the backtrace, it's hitting the WARN_ON() in the > > > > cmpxchg() loop below, because TASKLET_STATE is just > > TASKLET_STATE_CHAINED. > > > > It seems that the only way to turn off TASKLET_STATE_CHAINED is via > > this cmpxchg(), but TASKLET_STATE_RUN can be independently turned > > off > > elsewhere (tasklet_unlock() and tasklet_tryunlock()), so if that > > happens and this loop is hit, you could loop until loops runs out > > and > > hit this warning. > > But clearing TASKLET_STATE_RUN independently happens by the task, > that > set it / part of tasklet_schedule(). > tasklet_tryunlock() does a cmpxchg() with only the RUN bit so it > won't > work if the additional CHAINED bit is set. > > The tasklet itself (which may run on another CPU) sets the RUN bit at > the > begin and clears it at the end via cmpxchg() together with the > CHAINED > bit. > > I've been staring at it for sometime and I don't see how this can > happen. > I did find a problem with the patch when configured as !SMP since in that case the RUN flag is never set (will send a patch for that shortly), but that wouldn't be the case here. It would help to be able to reproduce it, but I haven't been able to yet. Tom > Sebastian
