Date: Mon, 5 Jan 2015 01:08:48 +0300 From: Gleb Smirnoff <glebius@FreeBSD.org> To: Hans Petter Selasky <hselasky@FreeBSD.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org, avg@FreeBSD.org Subject: Re: svn commit: r276626 - head/sys/kern Message-ID: <20150104220848.GU15484@FreeBSD.org> In-Reply-To: <201501031721.t03HLKHp060964@svn.freebsd.org> References: <201501031721.t03HLKHp060964@svn.freebsd.org>
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Hi! On Sat, Jan 03, 2015 at 05:21:20PM +0000, Hans Petter Selasky wrote: H> Author: hselasky H> Date: Sat Jan 3 17:21:19 2015 H> New Revision: 276626 H> URL: https://svnweb.freebsd.org/changeset/base/276626 H> H> Log: H> Rework r276532 a bit. Always avoid recursing into the console drivers H> clients, hence they might not handle it very well. This change allows H> debugging mutex problems with kernel console drivers when H> "debug.witness.skipspin=0" is set in the boot environment. H> H> MFC after: 1 week I'd ask Andriy avg@ to review r276532 and this change. AFAIR, he attacked this problem in 2011/2012 and it appeared much more complex than just adding MTX_RECURSE flag. H> Modified: H> head/sys/kern/kern_cons.c H> H> Modified: head/sys/kern/kern_cons.c H> ============================================================================== H> --- head/sys/kern/kern_cons.c Sat Jan 3 16:48:08 2015 (r276625) H> +++ head/sys/kern/kern_cons.c Sat Jan 3 17:21:19 2015 (r276626) H> @@ -512,6 +512,13 @@ cnputs(char *p) H> int unlock_reqd = 0; H> H> if (use_cnputs_mtx) { H> + /* H> + * NOTE: Debug prints and/or witness printouts in H> + * console driver clients can cause the "cnputs_mtx" H> + * mutex to recurse. Simply return if that happens. H> + */ H> + if (mtx_owned(&cnputs_mtx)) H> + return; H> mtx_lock_spin(&cnputs_mtx); H> unlock_reqd = 1; H> } H> @@ -601,13 +608,7 @@ static void H> cn_drvinit(void *unused) H> { H> H> - /* H> - * NOTE: Debug prints and/or witness printouts in console H> - * driver clients can cause the "cnputs_mtx" mutex to H> - * recurse. Make sure the "MTX_RECURSE" flags is set! H> - */ H> - mtx_init(&cnputs_mtx, "cnputs_mtx", NULL, MTX_SPIN | H> - MTX_NOWITNESS | MTX_RECURSE); H> + mtx_init(&cnputs_mtx, "cnputs_mtx", NULL, MTX_SPIN | MTX_NOWITNESS); H> use_cnputs_mtx = 1; H> } H> H> -- Totus tuus, Glebius.
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