Date: Tue, 11 Jan 2005 10:08:13 -0500 From: John Baldwin <john@baldwin.cx> To: Peter Jeremy <PeterJeremy@optushome.com.au> Cc: Scott Long <scottl@freebsd.org> Subject: Re: bus_dmamem_alloc() can't handle large NOWAIT requests Message-ID: <9D0B94DA-63E2-11D9-A25E-000393921CC4@baldwin.cx> In-Reply-To: <20050109101529.GM39552@cirb503493.alcatel.com.au> References: <20050109011132.GJ39552@cirb503493.alcatel.com.au> <41E0C02F.60100@freebsd.org> <20050109101529.GM39552@cirb503493.alcatel.com.au>
next in thread | previous in thread | raw e-mail | index | archive | help
On Jan 9, 2005, at 5:15 AM, Peter Jeremy wrote: > On Sat, 2005-Jan-08 22:25:03 -0700, Scott Long wrote: >>> According to bus_dma(9), bus_dmamem_alloc() can be invoked with a >>> flag BUS_DMA_NOWAIT to indicate that sleep()ing is not allowed. >>> >>> At least on the i386, if the requested size exceeds 1 page (or some >>> other cases), the requested memory will be allocated via >>> contigmalloc(). > ... >> Will contigmalloc() actually sleep? If so, then this is something >> that >> needs to be addressed in contigmalloc. > > I have not actually seen this occur. If I get some time, I might see > if I can force the issue. Reading the code, I believe it can. > > Firstly: > 1) if (vm_old_contigmalloc), contigmalloc1() locks vm_page_queue_mtx > via vm_page_lock_queues(). > 2) Otherwise, contigmalloc2() calls vm_map_lock() which locks > kernel_map->system_mtx > > Both these mutexes are MTX_DEF (sleepable) and there doesn't appear > to be anything preventing them from sleeping. Unforunately, "sleep" is a bit of an overloaded term in the kernel. Blocking on a mutex is not considered a "sleep" quite like msleep() or cv_wait() in that a mutex will eventually be released (barring an infinite loop bug) and thus a thread blocked on a lock won't sleep forever. msleep() and cv_wait() on the other hand do not have the same guarantee. M_NOWAIT means that msleep() and cv_wait() won't be called; it is ok to block on a mutex with M_NOWAIT however. I tried to explain this difference in semantics some in the glossary of the SMP chapter in the kernel devbook. > -- John Baldwin <jhb@FreeBSD.org> <>< http://www.FreeBSD.org/~jhb/ "Power Users Use the Power to Serve" = http://www.FreeBSD.org
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?9D0B94DA-63E2-11D9-A25E-000393921CC4>