Date: Mon, 30 Jul 2018 11:31:55 +0200 From: Roger Pau =?utf-8?B?TW9ubsOp?= <royger@freebsd.org> To: Pratyush Yadav <pratyush@freebsd.org> Cc: FreeBSD-Xen <freebsd-xen@freebsd.org>, Akshay Jaggi <jaggi@freebsd.org>, Edward Napierala <trasz@freebsd.org> Subject: Re: Recursion in non-recursive mutex when using the grant table free callbacks Message-ID: <20180730093155.c6ihgafia5ducw65@mac.bytemobile.com> In-Reply-To: <CA%2BX=3T%2BnHO6_Uhx=HFhcjQGe=GF_yOyEnO_nm6GS%2Bu7wfXR5qg@mail.gmail.com> References: <CA%2BX=3T%2BnHO6_Uhx=HFhcjQGe=GF_yOyEnO_nm6GS%2Bu7wfXR5qg@mail.gmail.com>
next in thread | previous in thread | raw e-mail | index | archive | help
On Sun, Jul 29, 2018 at 06:08:56PM +0530, Pratyush Yadav wrote: > Hi, > > Currently, the grant table free callbacks can not work. This is > because of a recursion on a non-recursive mutex that causes a kernel > panic. The cause of the recursion is: check_free_callbacks() is always > called with the lock gnttab_list_lock held. So, the callback function > is called with the lock held. So, when the client uses any of the > grant reference allocation methods get_free_entries() is called, which > tries to acquire gnttab_list_lock(grant_table.c:77 [0]), causing a > recursion on the lock. > > I'm not sure what the correct fix would be though. One way I can think > of is that check_free_callback() should be called without the lock > held. But with this fix, it is possible for the callback to be called > even though the grant references it needs are not available. This > would happen when another thread takes those references while the > current thread has completed the check if(gnttab_free_count >= > callback->count) but has not yet called the callback > (grant_table,c:105 [1]). > > I think a better way to fix this would be to have a check in > get_free_entries() whether the current thread holds the lock, so it > does not try to acquire the lock if the current thread already holds > it. I agree in the analysis, however I think the proper solution is to use a recursive lock. Roger.
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?20180730093155.c6ihgafia5ducw65>