panic(9) vs. RB_NOSYNC

[Konstantin Belousov]

On Thu, Aug 08, 2013 at 02:36:51PM +0900, Masao Uebayashi wrote:
> panic(9) (actually vpanic()) sets RB_NOSYNC when panicstr is already
> set.  What is the reasoning of this?
> 
> My understanding is that panic() attempts VFS "sync" operation at
> first.  If another panic() is triggered during that, give up VFS
> "sync".  Is this correct?  If so, how reliable is this design?  I
> wonder if attempting such a complex task like VFS "sync" after a panic
> is a good idea.

Look at the end of the vpanic(), right before kern_reboot() call.
There, kernel does
	if (!sync_on_panic)
		bootopt |= RB_NOSYNC;
so it only syncs when sync_on_panic sysctl is set to non-zero, which
is zero by default.

Basically, this option might be reasonable when you debug something
not related to i/o+VM+storage drivers, and you know what you do.
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