On Thu, Jan 19, 2017 at 07:59:04AM -0500, James Bottomley wrote: > On Thu, 2017-01-19 at 14:25 +0200, Jarkko Sakkinen wrote: > > On Wed, Jan 18, 2017 at 03:48:09PM -0500, James Bottomley wrote: > > > In a TPM2, sessions can be globally exhausted once there are > > > TPM_PT_ACTIVE_SESSION_MAX of them (even if they're all context > > > saved). > > > The Strategy for handling this is to keep a global count of all the > > > sessions along with their creation time. Then if we see the TPM > > > run > > > out of sessions (via the TPM_RC_SESSION_HANDLES) we first wait for > > > one > > > to become free, but if it doesn't, we forcibly evict an existing > > > one. > > > The eviction strategy waits until the current command is repeated > > > to > > > evict the session which should guarantee there is an available > > > slot. > > > > > > On the force eviction case, we make sure that the victim session is > > > at > > > least SESSION_TIMEOUT old (currently 2 seconds). The wait queue > > > for > > > session slots is a FIFO one, ensuring that once we run out of > > > sessions, everyone will get a session in a bounded time and once > > > they > > > get one, they'll have SESSION_TIMEOUT to use it before it may be > > > subject to eviction. > > > > > > Signed-off-by: James Bottomley < > > > james.bottom...@hansenpartnership.com> > > > > I didn't yet read the code properly. I'll do a more proper review > > once I have v4 of my patch set together. This comment is solely > > based on your commit message. > > > > I'm just thinking that do we need this complicated timeout stuff > > or could you just kick a session out in LRU fashion as we run > > out of them? > > > > Or one variation of what you are doing: couldn't the session that > > needs a session handle to do something sleep for 2 seconds and then > > take the oldest session? It would have essentially the same effect > > but no waitqueue needed. > > > > Yeah, as I said, this is just commentary based on the description. > > If you don't have a wait queue you lose fairness in resource allocation > on starvation. What happens is that you get RC_SESSION_HANDLES and > sleep for 2s and retry. Meanwhile someone frees a session, then next > user grabs it while you were sleeping and when you wake you still get > RC_SESSION_HANDLES. I can basically DoS your process if I understand > this. The only way to make the resource fairly allocated: i.e. the > first person to sleep waiting for a session is the one who gets it when > they wake is to make sure that you wake one waiter as soon as a free > session comes in so probabalistically, they get the session. If you > look, there are two mechanisms for ensuring fairness: one is the FIFO > wait queue (probabalistic) and the other is the reserved session which > really ensures it belongs to you when you wake (deterministic but > expensive, so this is only activated on the penultimate go around). > > James
Right, I see your point. /Jarkko ------------------------------------------------------------------------------ Check out the vibrant tech community on one of the world's most engaging tech sites, SlashDot.org! http://sdm.link/slashdot _______________________________________________ tpmdd-devel mailing list tpmdd-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/tpmdd-devel
