Date: Sun, 14 Dec 2008 21:42:45 -0800 From: Xin LI <delphij@delphij.net> To: Ravi Murty <ravi.murty@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: td_critnest Message-ID: <4945EE55.2050401@delphij.net> In-Reply-To: <95b10a340812142103u3ab8bd2br97033a7c7e8bec3@mail.gmail.com> References: <95b10a340812142103u3ab8bd2br97033a7c7e8bec3@mail.gmail.com>
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-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Ravi Murty wrote: > Hello All, > > The implementation of critical_enter and critical_exit changed between > freebsd 5 and freebsd 6. In the newer implemtnation, the code checks if > td_critnest is 1 and if it is sets it to zero, then checks if the thread > owes a preempt. If so, it increments td_critnest by 1 before grabbing a lock > and then decrements it back to zero. I can't figure out why it does this. > The freebsd 5 implementation seems straightforward where we check if the > thread owes a preempt and if so we switch to the new thread. Can anyone help > me with this? My guess is to prevent race conditions (i.e. to split the owepreempt flag into a separate variable), since this value could be changed in an interrupt context, and not only during the current thread context. Cheers, - -- Xin LI <delphij@delphij.net> http://www.delphij.net/ FreeBSD - The Power to Serve! -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.9 (FreeBSD) iEYEARECAAYFAklF7lUACgkQi+vbBBjt66C0sQCeOZCFZu4VBTRk3it4/424pAbc LRoAoLfdoS09ZX2SSZ1Z/SOw+rqkrkQ0 =P2Ez -----END PGP SIGNATURE-----
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